Boosting reactivity of water-gas shift reaction by synergistic function over CeO2-x/CoO1-x/Co dual interfacial structures

Dual-interfacial structure within catalysts is capable of mitigating the detrimentally completive adsorption during the catalysis process, but its construction strategy and mechanism understanding remain vastly lacking. Here, a highly active dual-interfaces of CeO2-x/CoO1-x/Co is constructed using the pronounced interfacial interaction from surrounding small CeO2-x islets, which shows high activity in catalyzing the water-gas shift reaction. Kinetic evidence and in-situ characterization results revealed that CeO2-x modulates the oxidized state of Co species and consequently generates the dual active CeO2-x/CoO1-x/Co interface during the WGS reaction. A synergistic redox mechanism comprised of independent contribution from dual functional interfaces, including CeO2-x/CoO1-x and CoO1-x/Co, is authenticated by experimental and theoretical results, where the CeO2-x/CoO1-x interface alleviates the CO poison effect, and the CoO1-x/Co interface promotes the H2 formation. The results may provide guidance for fabricating dual-interfacial structures within catalysts and shed light on the mechanism over multi-component catalyst systems.


Catalyst preparations
Preparation of CeO2 nanoparticles.The preparation procedure referred to the research reported before. 1 Typically, 6 mmol of cetyltrimethylammonium bromide (CTAB) was dissolved in deoxidized water (200 mL), followed by introducing 10 mmol Ce(NO3)3•6H2O inside.Subsequently, 0.2 molL −1 NaOH was dropwise added to the pre-mixed solution with vigorously stirring until the pH value reach 9.The suspension solution was further aged at 90 C for another 3 h.After filtering and washing, the as-formed precipitate was dried at 110 C in an oven for 12 h.Subsequently, the resultant powders were calcined at 400 C for 5 h in the air (5 C/min).

Scanning electron microscope (SEM).
The SEM images were taken on a Zeiss SUPRA55 scanning microscope with an acceleration voltage of 5.0 kV.

X-ray photoelectron spectroscopy (XPS) analysis.
Ten peaks were fit to the Ce 3d XPS spectrum and labeled according to Burroughs formalism, including five spin−orbit split doublets split by approximately 18.4 eV (v: 3d3/2; u: 3d5/2).For fitting purposes, the area intensity ratio I(3d5/2)/I(3d3/2) was fixed to 1.5 for each doublet pair.The u and v peaks located at about 902.4 eV and 884.0 eV result from a Ce3d 9 4f 1 O2p 6 final state.The lowest binding energy states u 0 and v 0 located at 898.4 eV and 880 eV result from Ce3d 9 4f 2 O2p 5 .An approximated calculation of the percent composition of Ce 3+ species was obtained according to the following equation.
Surface area of catalysts.Builder SSA-4200 physisorption analyzer was used in measuring the surface area of catalysts.The catalysts were degassed at 200 °C for 6 h under vacuum before measurement.
Surface area of each sample was calculated by the Brunauer-Emmett-Teller method.

X-ray diffraction (XRD).
The ex-situ XRD patterns were obtained by a PANalytical X'pert3 powder diffractometer (40 kV, 40 mA, λCu-Kα= 0.15418 nm) with an acquisition time of 8.5 min in the range of 10−90.The in-situ XRD patterns were obtained from the same machine with an Anton Paar XRK-900 reaction chamber.Samples were loaded in a ceramic sample holder with a diameter of 10 mm and a depth of 1 mm.The in-situ reaction camber was heated from room temperature to 600 C (interval: 100 C) with a ramping rate of 30 C/min under 5%H2/Ar (30 mL/min).Two rounds of measurements, each lasting for 20 min, were carried out for each selected temperature.The second measurement round was collected and used to determine the structure of the catalysts.
Temperature Programmed Surface Reaction (TPSR).For both catalysts, the pretreated procedure was the same as for the catalytic tests above.After activation, samples were pre-hydroxylated with ~3H2O/Ar (30 mL/min) followed by Ar purging at 250 °C for another 30 min to purge the adsorbed H2O.Subsequently, the gas was switched from Ar to 2%CO/Ar (30 mL/min) and then heated from room temperature to 400 °C (ramping rate: 5 °C/min).The outlet gases were analyzed by an online mass spectrometer (LC-D200M, TILON) with m/z = 28 (CO), 44 (CO2), 2 (H2).

Apparent activation energy (Ea), Apparent kinetic orders, and kinetic isotopic effect (KIE).
Appropriate amounts of catalysts diluted with SiO2 were used in the kinetics experiments.The apparent activation energy and kinetic orders were performed on the same fixed-bed flow reactor mentioned above, with CO conversions in the range of 5%−15%.To measure the reactants (CO and H2O) reaction orders of catalysts, the concentration of CO and H2O were varied from 0.5%−10% with the reaction conversion remaining in the kinetic regime.The kinetic isotopic effect of catalysts was measured under a steady state at 250 °C, where feed gas alternatively changed from 2%CO/12%D2O/N2 to 2%CO/12%H2O/N2.The outlet gas was analyzed with the IR gas analyzer (Gasboard-3500, Wuhan Sifang Company, Wuhan, China).

Steady-state isotopic transient kinetic analysis (SSITKA).
The SSITKA tests were measured in a selfconducted reactor, in which the inner diameter of the quartz reactor is 6 mm.A six-way valve was used to avoid the unnecessary signal perturb from gas switching.After the same activation pretreatment with catalytic tests, the catalysts were firstly subjected to 0.5% 12 CO/He with a space velocity of 120,000 mLgcat −1 h −1 in the absence of H2O at 250 C.After the signal reached a steady state, the gas flow was changed from 0.5% 12 CO/He to 0.5% 13 CO/He.Similarly, the isotopic exchanges in the presence of H2O were conducted with the same procedures, in which the feeding gas of 0.5% 12 CO/3%H2O/He was replaced by 0.5% 13 CO/3%H2O/Ar.The reversibly adsorbed CO was derived from the in-situ 12 CO/He to 13 CO/Ar switches at 250 C, which was representative of the total number of active sites (Ntotal).Subsequently, the adsorbed amount of CO (NCO) under steady state was calculated by the in-situ switches between 12 CO/H2O/He to 13  DFT Calculations.All DFT calculations were performed with the plane-wave basis sets of 400 eV cutoff kinetic energy to approximate the valence electron densities and projector-augmented wave method to account for the core-valence interaction, 3 as implemented in the Vienna Ab initio Simulation Package (VASP) code. 4,5DFT + U method with U = 5 eV and 3.7 eV were used to describe the localized Ce 4f and Co 3d states, respectively. 6The spin-polarized method with Perdew-Burke-Ernzerhof (PBE) flavor of generalized gradient approximation was employed. 7Gamma centered Monkhorst−Pack (111) sampling was used for the Brillouin zone integration for all computational models due to the large cell.The convergence criteria for energy and force were set as 10 −5 eV and 0.02 eV Å −1 , respectively.Transition states searched by climbing image nudged-elastic-band (CI-NEB) method with convergence criterion of 0.05 eV Å −1 . 8Vibrational analyses were further performed to ensure the local minimum and transition states and vibrational frequencies.
CO/H2O/Ar.The corrected averaged residence time () of CO was calculated based on the integrated area of the evolution curve detected by MS ( 12 CO or 13 CO; m/z = 28 or 29) excluding the corresponding integrated area of inert gas (He or Ar; m/z = 4 or 40).  = ∫   () − ∫  ()The number of surface adsorbed species (Ni) was calculated as the following equation, where the F is the exit flow of CO:  =   × The coverage of CO was calculated based on the following equation:  =    Computational model.CoO(100) surface was chosen in our work due to its high stability among the low-index ceria surfaces.CoO(100) was modeled by p(4 × 4) 4-layer supercells with the top two layers relaxed, and with a vacuum gap between slabs at ∼15 Å. CeO2-x is represented by a typical Ce6O13 cluster reported in previous work.2Co 0 /CoO(100) model is represented by a Co10 metal cluster on CoO(100).